1. Field of the Invention
The present invention relates to an overcurrent protection circuit for controlling an output current of a voltage regulator configured by semiconductor integrated circuit.
2. Description of the Related Art
FIG. 2 is a circuit diagram showing a configuration of a conventional foldback overcurrent protection circuit corresponding to a voltage regulator constituted by a depletion transistor as an output transistor. When output current flows through the depletion output transistor 4, detection current also flows through a foldback overcurrent protection circuit 8 accordingly.
When a level of the detection current reaches a predetermined value which is set inside the foldback overcurrent protection circuit 8, the foldback overcurrent protection circuit 8 starts to control the output current flowing through the depletion output transistor 4. In this case, a source voltage becomes the output voltage since the depletion output transistor 4 operates as a source follower. In order that the output current vs. output voltage characteristic curve should show a foldback characteristic, a gate voltage of the depletion output transistor 4 must be lower than the output voltage. In addition, when the output voltage is at the GND level, in order to further reduce the output current, the gate voltage needs to be made negative. However, in the conventional foldback overcurrent protection circuit, it is difficult to reduce the output current flowing through the depletion output transistor since the depletion output transistor is controlled by a circuit which operates at the input voltage VDD and whose reference voltage is GND by using a detection voltage within the output voltage ranging between input voltage VDD and GND.
The solid line in FIG. 3 shows the output current vs. output voltage characteristic in overcurrent detection state by the foldback overcurrent protection circuit corresponding to the voltage regulator constituted by a depletion output transistor. The dotted line shows a curve which is required for a foldback protection circuit and which has already been implemented in a regulator constituted by an enhancement mode output transistor. As shown in the figure, the output current increases from a point “v” on a characteristic curve before detection of overcurrent and at the knee point overcurrent is detected. When overcurrent is detected, the output current decreases. Through a point “b”, however, the output current increases to a point “c” on the characteristic curve, not to a point “a”. Thus, the output current is not reduced, but increases (refer to JP 7-74976 B for example).
Heretofore, the foldback overcurrent protection circuit for the voltage regulator constituted by a depletion mode output transistor has a disadvantage in that it is difficult to control the output current vs. output voltage characteristic to show an ideal foldback characteristic because the output voltage ranging between VDD and GND is utilized for the detection voltage for the overcurrent protection circuit.